Money once in Europe set aside (6)
I believe the answer is:
peseta
'money once' is the definition.
(former Spanish currency)
'in europe set aside' is the wordplay.
'in' says the answer is hidden in the clue.
'PESETA' is hidden in the letters of 'europe set aside'.
(Other definitions for peseta that I've seen before include "Spanish coinage" , "currency being superseded" , "Pre-Euro Spanish currency" , "Get ready once for Spain" , "Spanish bread once" .)